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CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability

criticalknown exploitedpublic exploitCVE-2026-21509
Actively exploited. At least one CVE in this advisory is listed in the CISA Known Exploited Vulnerabilities catalog — exploitation has been observed in the wild. Treat remediation as urgent.
Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.

CSIRTS triage

vendor: Microsoftproduct: OfficeOther
What
A security feature bypass vulnerability could allow unauthorized attackers to bypass security features locally.
Who is affected
Users of Microsoft Office are affected, especially those on end-of-life versions.
Urgency
Remediation is critical due to active exploitation and the potential for unauthorized access.
Action
Transition to a supported version of Microsoft Office.

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Details

Source
CISA Known Exploited Vulnerabilities (US · database · site)
Severity
critical
Published
2026-01-26
Exploitation
Observed in the wild (CISA KEV)

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-21509

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Referenced CVEs

CVECSIRTS overviewExternal
CVE-2026-21509coverage & exploitation statusNVD · CVE.org

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